The commercial success of pressure-sensitive adhesive (PSA) tapes which utilize acrylate or acrylic polymer or copolymer adhesives as the viscoelastic adhesive layer is well known. Ulrich, U.S. Pat. No. Re. 24,906, discloses acrylate polymer adhesives which have the well-known critical four-fold balance of physical properties needed for a successful PSA composition, vis-a-vis, adhesion, cohesion, stretch and adhesive tack. Despite the commercial success of acrylate polymer PSA, residual monomer in the final polymerized product is a problem which has been for the most part been tolerated. In today's market, the criteria for various products which include such adhesive compositions have been raised for many applications to a point where conventional residual monomer levels have become unacceptable.
Copolymers of isooctyl acrylate or other high boiling acrylates with acrylic acid or acrylamide are representative PSA compositions which have monomer residue. Residues of such monomers are unwanted mainly because they give off unpleasant odors. Although polymerizations can be carried to extremes where conversions of monomer starting materials (hereafter referred to as primary monomer) to copolymer are well above 95%, it is almost impossible to eliminate all unreacted primary monomer during the initial reaction. And, such unreacted primary monomers are not easily removed by volatilization because of their low volatility.
The following prior art reveals various known methods of dealing with residual monomer:
Taylor, U.S. Pat. No. 4,529,753, teaches a process of treating an emulsion of one or more monomers, at least 80 weight percent of which have been polymerized, to reduce residual monomer in the resultant polymer. The process involves subjecting the emulsion to temperature and pressure conditions at which the vapor pressure of the water in the ambient environment is less than the vapor pressure of the water in the emulsion and introducing into the emulsion a free readical generator until the residual monomer content is reduced.
Chu et al., U.S. Pat. No. 4,476,262, disclose a process for preparing an aqueous dispersion composition which includes a water dispersible ionic polymer component and an addition polymer to have reduced amounts of extractable undesirable residue monomer that is used to prepare the addition polymer. The undesired monomer residue is reduced by conducting an additional polymerization step in the presence of addition polymer containing residual monomer with another monomer which is capable of copolymerizing with the undesired monomer and which results in a less undesirable monomer residue or a residue which is more easily removable. The ionic polymers are preferably based on epoxy resin.
Lindsey et al., U.S. Pat. No. 4,510,298, disclose a process for reducing residual toxic isocyanate monomer left after the polymerization is completed. The process is accomplished by use of a scavenger near the end of the polymerization reaction. The scavenger, an active polymerization monomer (preferably butyl acrylate), is added in an amount at least equal to the isocyanate monomer when at least 98% of the isocyanate monomer has been polymerized. Butyl acrylate is recognized by Chu et al. ('262) as giving off an objectionable odor. While the use of butyl acrylate may remove residual toxic isocyanate monomer, it could leave the polymer with an undesirable odor.
No disclosure is known of employing a scavenger to reduce undesirable monomer in acrylate polymer PSA compositions. Even if such a process modification were suspected of being useful for the production of acrylate polymer PSA composition, by its nature it includes the production of a copolymer of a scavenger monomer and unwanted residual monomer which could have an adverse effect upon the critical four-fold balance of physical properties needed for a successful acrylate PSA composition.